Generally, "bandgap" is a term used in physics and its related semiconductor technology. In physics, when the distance between two atoms approaches the equilibrium interatomic spacing of a diamond lattice, energy level splits into two bands. The two bands are separated by a region which designates energies that the electrons in a solid, such as a type of semiconductor material, cannot possess. The region is referred to a forbidden gap, or a bandgap, for this type of semiconductor material. Any change of thermal energy, electron or photon energy may affect the width of a bandgap. For example, any increase in temperature, electron or photon energy will tend to narrow a bandgap, and similarly, any decrease in temperature, electron or photon energy will tend to widen the bandgap. In addition, depending on the types of semiconductor materials, a bandgap can be wide for one type of material but narrow for another type. For example, silicon generally has a much wider bandgap than gallium arsenide (GaAs).
Many semiconductor devices, such as diodes, bipolar transistors, BiCOMs Field Effect Transistors (FETs) etc., have used bandgap characteristics of a particular semiconductor material, such as silicon. In these devices, such as a diode, a positive electrical charge can narrow the bandgap, and a negative electrical charge can widen the bandgap. In a certain operating region of a device or a circuit containing a plurality of these semiconductor devices, a bandgap can be wide enough such that a voltage at one point of the circuit is stable independent of an applied power supply. The stable voltage at that point is often used as a voltage reference, and a circuit used for designing such a stable reference voltage is often referred to as a bandgap voltage reference circuit.
In silicon bipolar, BiCOMs related technologies, a bandgap circuit employing bipolar transistors has been used to provide stable reference voltages for many years in semiconductor industry.
In recent years, gallium arsenide (GaAs) based semiconductor chips have become more and more utilized in semiconductor industry. In such GaAs chips, where bipolar transistors are not an option, it is generally difficult to design a bandgap circuit to provide a reference voltage which is independent of a power supply voltage of the circuit.
Based on the physics characteristics of the semiconductor materials described above, it is generally known that a bandgap voltage reference circuit (or in short, "a bandgap circuit") can be built from the exponential relation between the voltage and the current in an emitter junction of a bipolar transistor. It is also known that a Field Effect Transistor (FET) GaAs-based transistor exhibits a square-law relation between the voltage and the current. As a result, FET GaAs-based transistors generally do not meet requirements to build a bandgap reference circuit.
FIG. 1 illustrates a conventional bandgap circuit built from bipolar transistors Q1-Q4. The reference introducing this type of conventional bandgap circuit can be made to an article authored by A. P. Brokaw, published in IEEE Journal of Solid State Circuits, Vol. SC-9, pp. 388-393, December 1974, entitled "A Simple Three-Terminal IC Bandgap Reference". In this conventional bandgap circuit, the relation of the currents (I) and resistors (R) are as follows:
I0=I1; PA1 I2=I3=I4; PA1 R1=R2=R3; and PA1 R5=R6.
In addition, an amplifier, AMP, has two inputs that are connected to nodes n1 and n2, respectively. With appropriate values of the resistors, the amplifier, and the bipolar transistors, the bandgap circuit makes use of the fixed voltage difference between the base and the emitter of the bipolar transistors, which operate at different current densities, to produce a stable output voltage Vout at node n0, i.e. the bandgap circuit output Vout or Vn0 is independent of a power supply voltage Vdd of the amplifier. Thus, the stable Vn0 is used as a reference voltage.
Accordingly, there is a need for a bandgap circuit built from FET GaAs-based transistors to provide a stable reference voltage which is independent of a power supply voltage of the circuit.